1. Field of the Invention
The present invention relates to a bobbin thread cutting device of a sewing machine for cutting a bobbin thread extending out of a bobbin case provided within a bed of the sewing machine, and more specifically, to a bobbin thread cutting device capable of cutting a bobbin thread so that an extending portion of the bobbin thread will be of a predetermined length necessary for forming a first stitch in sewing operations, and temporarily anchoring an end portion of the cut thread as a result of cutting.
2. Description of the Prior Art
Conventionally, a preparatory operation is needed before performing sewing operations. That is, after inserting a bobbin case with a bobbin holding a bobbin thread into a loop taker, a needle with a needle thread stitches once without cloth on a bed. Then, when an end of the needle thread is pulled, the bobbin thread is drawn out of a needle hole of the needle plate. Also, at this time, a needle thread and the bobbin thread are excessively drawn out.
Japanese Patent Publication No. SHO-56-20876 describes a sewing machine with which no preparatory operation to draw out the bobbin thread is needed. As shown in FIG. 1, the sewing machine includes a needle 113 maintained vertically and movably up and down, a bed 111, a loop taker 118 positioned within the bed, a bobbin case 119 removably mounted inside the loop taker 118, a bobbin 120 holding a bobbin thread 102 and removably mounted in the loop taker 118, and a bed slide 121 provided on an upper surface of the bed 111 near the loop taker 118. The loop taker 118 forms lock stitches in cooperation with the needle using the bobbin thread 102 and a needle thread (not shown). A user can slide open the bed slide 121 to access the bobbin 120 and/or bobbin case 119 for replenishing or replacing the bobbin thread 102.
A cutting blade 122 for cutting the bobbin thread 102 is installed on a free end of the bed slide 121. Also, a bobbin thread cutting assist member 124 is provided in the bed 111 so as to be positioned near a blade edge 122a of the cutting blade 122 when the bed slide 121 is in a closed position. A midway portion of the bobbin thread 102 pulled out of the bobbin case 119 is bent into an approximate Z-shape around the cutting blade 122 and the bobbin thread cutting assist member 124.
With this configuration, when the bed slide 121 is closed after the bobbin case 119 with the bobbin 120 has been inserted to the loop taker 118, the bobbin thread 102 issuing from the bobbin case 119 is easily cut to a certain length necessary for forming a first stitch. It is unnecessary to draw out the bobbin thread 102 in preparatory operations using a needle thread, and the sewing operations can be begun immediately after inserting the bobbin case 119. Also, a length of a needle thread extending from the needle can be shortened, and a neat stitch can be formed.
However, a direction in which the bobbin thread 102 extends out of the bed 111 is approximately in parallel with a direction in which the thread 102 extends from the bobbin case 119 to the bobbin cutting assist member 124. Therefore, if there is little overlap in the parallel portions of is thread 102, the frictional resistance in the bent portions of the Z-shape is insufficient to cut the bobbin thread 102. Also, if the user pulls out the bobbin thread at this time, the thread 102 has a tendency to slip over the blade edge 122a and be pulled out too far before being cut. In addition, the end of cut bobbin thread 102 may fall inside the bed 111 of the sewing machine, and may be suspended from the loop taker 118.
On the other hand, if the cutting blade 122 and the bobbin thread cutting assist member 124 are positioned to provide a larger overlap in the parallel portions of the bobbin thread 102 to provide more acute bending angle in the Z-shaped portion of the thread 102, a risk of collision between a surface of the cutting blade 122 and an edge of the bobbin thread cutting assist member 124 when closing the bed slide 121 increases, thereby inviting damage to the cutting blade 122 and reducing durability thereof.